The blasting industry is involved in numerous activities such as mining, road construction, demolition and seismic exploration. The blasting industry provides the explosive materials and the skills required to perform work in these areas. The combustion, i.e. detonation, of the explosives results in the generation of high quantities of energy in a very short period of time. This energy is used to perform work such as earth excavation and rock fracturing. The proper utilization of explosives in these applications involves numerous methods and techniques. For example, a typical mining application requires a large amount of energy to break rock and move it into a recoverable location. In order to accomplish this, it is common practice to drill bore holes into the desired rock or ore bed. The bore hole is then charged with an initiation assembly which is lowered into the bottom of the bore hole. The initiation assembly contains a very small amount of explosive. The initiation assembly typically includes a blasting cap or detonating cord. The bore hole is then filled with a relatively insensitive main explosive charge e.g. blasting agent or high explosive. The quantity of main explosive charge is very large when compared to the initiation assembly and is designed to perform the desired explosive work, i.e. break the rock. To allow the main explosive charge to perform the desired work, energy is delivered to the initiating assembly causing it to combust or detonate and the output of the initiating assembly causes the main charge to detonate or combust.
In an effort to increase the safety associated with handling the large quantity of main explosives, the mining industry has developed main explosive charges that are inherently insensitive. This feature requires that the initiating assembly provide more energy to initiate the main charge. The incorporation of an explosive booster to the initiation assembly has been the common method to increase the energy available from the initiation assembly. The amplified output from the larger explosive booster charge in turn initiates the large volume of insensitive main explosive charge. Many explosive applications involve the use of cast boosters. The commercial cast booster, a.k.a. cast primer, explosive primer or booster, primer or booster, will be referred to as a booster for the remainder of this disclosure. In summary, the booster is an explosive charge that functions as a transfer charge between an initiator such as a blasting cap or detonating cord and an insensitive main explosive charge. The booster is designed to 1) be sensitive enough so that it can be initiated by an initiator such as a blasting cap or detonating cord, 2) detonate as a result of the initiation stimulus and 3) upon detonation, generate enough energy and power to initiate the relatively insensitive main charge.
The common booster is well known and typically comprises a shaped, sensitive explosive composition or assembly located within a casing, typically a paper, cardboard or plastic container. The shape is typically a mostly solid right cylinder with one or more apertures that are parallel to the axis of the cylinder. The length and diameter of the booster varies and is dependent on the specific end use. Commonly, the booster will be about 1-2 inches in diameter and about 3 to 5 inches in length.
Boosters typically have booster apertures that are designed to accept an initiating device. The two most common initiating devices are a blasting cap and detonating cord. The apertures may be located centrally or off center from the cylinder axis and vary in diameter but typically are between about 0.250 to 0.300 inches in diameter. The depth of the apertures also varies depending upon the design requirements. Commonly, one aperture runs end to end through the booster to accommodate the insertion of a detonating cord. The other aperture is a typically a blind cavity with a depth and diameter designed for the insertion and seating of a blasting cap. The aperture for the blasting cap is commonly called the cap well and is a blind hole with a diameter slightly larger than the blasting cap diameter. The aperture for the detonating cord is typically referred to as the detonator cord thru-hole and is designed to have a diameter that allows a detonating cord to pass through unrestricted. Additional apertures designed for other special initiating devices may also be present. In general, the apertures are referred to as the initiating apertures.
Because the two common types of initiating devices have differing dimensions, configuration and output strength, the booster is commonly designed with features that allow proper coupling of either initiating device to the booster. FIG. 1A depicts a basic booster 7 and FIG. 1B depicts a cross section of the basic booster. FIG. 1C depicts an isometric view of the basic booster. In the case of the blasting cap, a well (a.k.a. cap well) or blind cavity 1 is formed in the booster. For the detonating cord, a thru hole 2 is formed in booster. The basic booster construction is comprised of an enclosure formed by the bottom cap 5, the sleeve 3 and the top cap 4. The top cap 4 is optional. The materials of construction for these items are often cardboard or plastic. The enclosure is filled with the booster explosive 6.
FIG. 2A shows common initiation sources, i.e., a blasting cap 8 or detonating cord 10 that are used with the common booster 7 and FIG. 2B shows the assembly of the booster and initiating devices. When the blasting cap 8 is used as the source of initiation, the blasting cap 8 is inserted into the blind cavity 1. The output of the blasting cap is primarily from the blasting cap end due to the base charge explosive charge 9 inside the blasting cap. When the detonating cord 10 is used as the initiation source, the detonating cord is passed through the booster through the detonator cord channel 2.
FIG. 3 shows a common booster manufacturing method that is based on a melt-pour type casting operation. The main sheath explosive compositions 6 typically have melting points near 1750F. When heated beyond the melting point, the explosive composition becomes fluid. To form a booster, the fluid explosive is poured into containers that serve as a mold as well as a container or enclosure. As shown in FIG. 3A, the container, typically comprised of a bottom cap 5 and casing 3, is pre-assembled to form a cup. Typically, inserted through the bottom cap 5 are removable apertures forming pins 11 that serve to form the channels that will accept the initiating devices. As shown in FIG. 3B, the explosive 6 is poured into the container or enclosure. A top cap 4 with mating holes is often placed over the open end. FIG. 3C represents a cooling stage during which the molten explosive cools and hardens. As shown in FIG. 3D, once the explosive cools and solidifies, the pins 11 are removed from the container resulting in a jacketed solid cylindrical charge having the cap well and the detonator cord thru hole 7.
In field use, the booster design must account for the fact that the pre-set output strength of the initiating devices such as blasting caps or detonating cord must initiate the booster and cause it to detonate. To ensure that the initiating devices can detonate the booster, the booster is made of an explosive that is relatively sensitive such that it can be directly initiated by the blasting cap or detonator cord. One common explosive material that has the required sensitivity is a mixture of the sensitive granular explosive PETN and less sensitive and melt-able explosive TNT. The granular PETN is dispersed in the melted TNT to form the cast-able composition. This combination in certain ratios is commonly referred to as pentolite. However, pentolite is expensive and relatively hazardous to handle during both the booster manufacturing operation and in the field during bore hole loading operations. In addition, unless the process is well controlled, the PETN is able to settle in the liquid TNT resulting in a non-homogenous dispersion of the PETN in the TNT and cast boosters with varying degrees of sensitivity.
U.S. Pat. No. 3,037,452 issued to M. Cook on (Date) and entitled “Booster for Relatively Insensitive Explosives”, discloses a booster that involves a small core of sensitive explosive surrounded by less sensitive sheath explosive charge. The sensitive inner charge will herein be referred to as the core and the less sensitive surrounding explosive charge will be herein referred to as the sheath. FIGS. 4A, B, C, D & E depict the core/sheath concept. In accordance with the concept, the core 12 is comprised of a relatively sensitive explosive that is more sensitive than the sheath charge 13. The core is strategically located inside the sheath charge in close proximity to the cap well 1 and/or detonator cord thru-hole 2. Thus, in this configuration, the output of the initiating device initiates the core that in turn initiates the insensitive sheath explosive charge of the booster. This concept reduces the material costs of the booster by reducing the amount of sensitive explosive. Additionally, since the sheath is a less sensitive explosive, the hazards associated with handling the booster in the field are also reduced.
U.S. Pat. No. 3,037,453 discloses a core/sheath cast booster wherein the core is formed by a FIG. 8 configuration of detonating cord.
U.S. Pat. No. 3,604,353 discloses a cast booster assembly comprising a sheath made of two layers and a core. In this concept, a layer of TNT is topped by a layer of pentolite and the detonator cord sensitive core is primarily in contact with the pentolite layer.
U.S. Pat. No. 3,747,527 discloses a core/sheath cast booster that utilizes a cast pre-formed core with cylindrical and similar shapes that have contact with the sides of the initiating channels, i.e., the detonator cord thru hole or cap well.
U.S. Pat. No. 4,776,276 discloses a core/sheath cast booster which utilizes PETN in a sleeve surrounding the detonator cord initiation channel.
U.S. Pat. No. 4,945,808 discloses a core/sheath booster configuration that utilizes a core comprised a rigid waterproof container that contains a loose charge of a sensitive explosive such as PETN.
The core for most prior art adaptations of the core/sheath concept commonly takes the form of an uncompacted, loose charge of granular PETN that is housed in a cylindrical capsule or rubber bladder i.e. balloon. The core is then attached to the mold pins that form the initiating apertures using a tie or elastic band. The main sheath explosive charge is poured into the cardboard cast housing enveloping the core except where the charge is in direct contact with the pins. By design, the core and inner surface of the cap well and detonator cord thru-hole share a common surface, i.e., the core is positioned near the inner surface of the initiation apertures. This prior art method has several drawbacks including 1) the high production costs related to filling the balloons with the PETN and positioning and retaining the balloon about initiation channels, 2) the reduced initiation reliability related to the ability to properly position and retain the balloon in contact with the initiation channel, 3) the hazards associated with the handling of the dry PETN during the balloon filling process, 4) the hazards of handling the booster in the field due to the impact sensitivity of the dry, loosely compacted PETN, 5) the reliability of initiation signal transfer between the detonator and the core due to the variable core coupling with the either initiation apertures, 6) the lack of coupling between the core and the axial output from the blasting cap and 7) the low core output strength available to initiate the sheath explosive due to the use of core formation using a loose, low density explosive.
Another prior art method that is employed utilizes a cast core that has the shape of a cylinder that mates with the aperture forming pins. In this case, the casting is typically made using pentolite. In some cases, the casting is made on the aperture forming pins and once hardened, the sheath explosive is cast around the inner core casting. In other cases, the core is formed and then inserted onto the aperture forming pins and positioned and affixed along the aperture forming pin length. The drawbacks associated with this prior art method are 1) the formation of the core requires an additional casting process, 2) the core composition, pentolite, is relatively hazardous in handling and 3) the lack of coupling between the core and the axial output from the blasting cap.
It is well known in the industry that increasing the distance between a donor explosive, e.g. blasting cap or detonating cord, and an acceptor explosive, e.g. explosive core, will reduce the reliability of initiation or detonation transfer of the acceptor explosive by the donor. Since the output of either the blasting cap or detonating cord must transfer to the inner surface of their respective apertures to initiate the core, the surface area that is mutually shared by the core and the initiation aperture and the proximity of the core to the initiation aperture surface is directly related to the reliable initiation transfer between the initiator and the core. As the mutual surface area between the core and the initiation aperture increases, the transfer of the detonation from the initiator to the core becomes more reliable. Similarly, the closer the core is to the aperture inner surface, the greater the initiation transfer reliability.
The degree of coupling exhibited in the prior art is variable. FIGS. 7 A-I show prior art examples of coupling between the core and the apertures. In FIGS. 7 B & C, the PETN filled balloon core 16 coupling is erratic and, in FIGS. 7 D & E, the cylindrical core 17 has only tangential contact with the aperture inner surface. In FIGS. 7 F & G, the contoured type container or core 18 increases the radial contact with the apertures while, in FIGS. 7 H & I, the donut style core 19 maximizes the radial contact. In all cases, however, there is no axial coupling between the initiator in the blasting cap aperture and the core. Since the most powerful output from the blasting cap is directed axially from the bottom of the blasting cap, this lack of coupling reduces the reliability of detonation transfer between the blasting cap and the core.
The alignment of the core with the initiating apertures is equally critical to the proper function of a core-sheath style booster. The prior art core designs including a donut shape, a dog bone shape, a balloon, a straight capsule or a contoured capsule must be affixed to the molding pins prior to pouring of the sheath explosive. In particular, as related to the blasting cap aperture, the core must be positioned axially to match up with the output from the blasting cap base section. If the core is out of position, the output from the blasting cap will not initiate the core and, thus, the sheath also will fail to initiate.
FIGS. 8 A-E depicts assembly of a core-sheath booster 21 that utilizes a PETN filled balloon style core. In FIG. 8A, the core 16 is manually attached to the pins. This attachment is typically accomplished using an elastic band 20. Next, in FIG. 8B, the bottom cap 5-casing 3 is placed over the pins 11. As shown in FIG. 8C, the molten sheath explosive 13 is then poured into the enclosure. The end cap is attached to the casing and allowed to cool as shown in FIG. 8D. The core-sheath style booster 21 with the formed initiating apertures is removed from the pins 11 once cooled. In this example, a single balloon core must satisfy the coupling for both the blasting cap aperture and the detonator cord aperture. Thus, the core position must be precisely located between the two pins.
FIG. 8F depicts the range and direction that the core could be misaligned. The location of the core can be anywhere along the length of the pin per the Z range and any position around the pin per the path constrained by the X & Y range. In this case, as well as other prior art, the misalignment of the core is related to 1) operator error and 2) movement of the core in the X, Y and Z due to the force from the flowing molten explosive going into the casing and 3) core slippage in the Z direction due to the heating of the attached core. Movement of the explosive core in any of the X, Y or Z directions can increase the distance between the core and initiation source enough to cause detonation transfer failure. Even a small increase in distance can significantly reduce the reliability of detonation transfer from the initiator in the initiating aperture and the core.
One universal characteristic of the explosive core is that it must be sensitive enough to be initiated by a detonator cord or a blasting cap. As a result of the poor coupling design between the core and the initiation apertures or variable distance between the initiation apertures and the core, the core explosive must be relatively sensitive. The typical explosive materials used to make the core include PETN, RDX, tetryl and pentolite. Those skilled in the art of blasting recognize that the ability of the blasting cap or detonating cord to initiate the core explosive charge is related to the intrinsic sensitivity of the basic explosive compound as well as the density of the core explosive composition. Also, it is well known to those skilled in the art that a low density form of the granular explosive is typically more sensitive than a higher density form e.g. compressed or cast, to initiation by the output of a blasting cap or detonating cord. Also, it is well known that the output of the explosive is proportional to the square of the explosive density. In order to account for the poor core coupling and variable core position, yet still provide proper detonation transfer between the initiation source and the core, current practice relies on using a very sensitive core comprised of a loose, low density PETN charge. As a result and due to the lesser output from the low density form, the size of the core must be substantial to properly initiate the insensitive sheath explosive.
The hazards associated with manufacturing and handling explosives are well known. The severity of the hazard is dependent on the type and form of explosive. In practice, the common core-sheath booster designs utilize cores that are comprised of dry, un-compacted, granular PETN. In this form, the PETN is relatively sensitive to inadvertent initiation due to an impact or friction. Thus, handling of PETN during manufacturing is relatively hazardous. Also, the final booster assembly containing the loose PETN core is more susceptible to inadvertent initiation from sources of impact such as are found in a blasting environment.
Prior art cast shaped cores, such as a pentolite explosive admixture which is a melt pour mixture of PETN and TNT, are also relatively hazardous to process and handle due to the inherent sensitivity of PETN. A formed core made with a cast-able composition such as pentolite also requires a separate casting operation. Thus, in order to form the booster, two laborious casting operations are required, one for the core and one for the booster.
The formation of loosely compacted PETN cores requires a loading process that dispenses the PETN in discrete amounts into a charge holder such as a balloon or capsule. This type of loading process is typically a slow laborious operation. The booster assembly operation is based on a melt pour casting type operation. In order to locate the loosely compacted PETN core within the booster, the PETN container must be manually attached to the aperture molding pins. This is commonly accomplished manually using an elastic band. This operation requires significant manual labor. The use of the aforementioned shaped cores also requires process design considerations and related assembly operations to ensure that core is properly positioned. This typically involves increased labor to carry out and ensure the proper positioning. In addition, booster materials costs are driven by the size of the core and the composition of the sheath. Due to the low output of the loose granular explosive core and limited and unpredictable coupling between the initiating apertures and the core, the core explosive quantity must be sized in excess to account for the worst case conditions.
Due to the low output of a loose granular explosive core, the sheath explosive composition must be formulated to have an appropriate sensitivity. The common method to increase the sensitivity of the sheath is to add in specific ratios loose granular explosives such as PETN, RDX or HMX to the melt pour base explosive (typically TNT). Thus, for a low output strength core, such as loose PETN, the sheath must contain a greater amount of the sensitizing granular explosives. Since the granular explosives cost is significantly greater than the TNT base explosive, the cost of the sheath material will increase as a result of a lower strength core.
Therefore, in view of the above discussed limitations of prior art boosters, what is needed is a safe and reliable booster for initiating a main explosive charge that is inexpensive to manufacture and use.